The present invention relates to phase detectors and, more specifically, relates to phase detectors having a filter for reducing systematic and pattern dependent jitter.
In pulse amplitude modulation (PAM) communication systems, precise symbol timing is essential for reliable performance. Generally, error in circuits for recovering symbol timing is due to temperature drift, aging, and misadjustment. To combat these problems and to correct timing error automatically, a timing recovery circuit such as a phase locked loop may include a sampling unit and a digital timing phase detector.
For digital timing phase detectors, a main source of error in symbol timing is due to systematic or pattern-dependent jitter. Generally, systematic or pattern-dependent jitter occurs as a result of a signal stream of symbols having a sequence or pattern with few signal transitions (e.g., the signal is not periodic in nature). A typical prior art phase detector receiving a completely periodic signal stream (e.g., a clock signal) provides highly accurate timing information. However, for a non-periodic or partially periodic signal stream, the typical prior art detector provides much less meaningful timing information. In multilevel PAM communication systems, systematic or pattern-dependent jitter is specially problematic due to the inherently fewer zero-transitions of a PAM signal stream.
With reference to FIG. 1, in one prior art phase detector for detecting symbol timing by a wave differential method, also known as the Gardner phase detector, the phase detector 20 receives a signal stream of pulse-shaped symbols. Sampler 24 samples the signal stream at twice the symbol rate where fs is the symbol rate. The three most recent samples are processed and re-sampled at the symbol rate to provide an output signal indicating symbol timing error.
In operation, the first sample in a set of three most recent samples is subtracted from the last sample in the set. The resulting subtracted signal is multiplied by the middle sample to provide an output signal. If the received signal stream of pulse-shaped symbols has zero timing error, the average output of the phase detector 20 is zero. However, even under such ideal conditions the variance of the phase detector output signal is not zero. The non-zero variance is due to the intermediate samples being dependent on adjacent symbols and hence contributing to systematic or pattern-dependent jitter.
With reference to FIG. 2, in another prior art technique, phase detector 30 may include a signal estimator 32 and a subtractor 34. The estimator 32 estimates the influence of adjacent symbols on intermediate samples. The subtractor 34 subtracts the output of the estimator 32 from the intermediate samples to reduce the effect of systematic and pattern dependent jitter on the output signal of the phase detector 30.
Accordingly, it is an object of the present invention to provide a novel phase detector including a filter for filtering a received signal stream of symbol before symbol timing is extracted.
It is another object of the present invention to provide a novel filter for operating on a received signal stream of pulse-shaped symbols to thereby provide a filtered received signal to a phase detector for reducing pattern dependent jitter of the phase detector.
It is yet another object of the present invention to provide a novel PAM communication system having a receiver including a phase detector for providing accurate symbol timing information with less pattern-dependent jitter.
It is still another object of the present invention to provide a novel receiver including a phase detector having a filter for increasing the periodicity of a received signal stream of pulse-shaped symbols before symbol timing is extracted from the signal stream.
It is a further object of the present invention to provide a novel method for reducing pattern-dependent jitter in the output of a phase detector.
It is yet a further object of the present invention to provide a novel phase locked loop having a filter for filtering a received signal stream of pulse-shaped symbols as an initial stage before phase detection.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.